Polymerization inhibitor



2,915,142 'POLYMERIZATION INHIBITOR Oscar A. Sarabia, Texas City, Tex., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware N Drawing. Application December 27, 1957 Serial No. 705,479

4 Claims. (Cl. 183-115) --p o sitio n or dissociation, or by partial oxidation, the acetyl cue-containing gaseous mixture produced generally containsso-called poly-formers, that is, compounds of the nature of biacetylene, for example, which polymerize to solid materials under certain conditions of temperature, pressure, etc. The purification of such an acetylenic gas mixture is usually accomplished by the selective action of suitable solvents. A number and variety of solvents have been proposed among which may be mentioned, for

example,.water, acetone, dimethylformamide, butyrolac- 1 tone, N-methyl pyrrolidone, triethyl' phosphate, and diethyl carbonate. The normal procedure for separation and recovery of acetylene from gas mixtures by means .pfabsOrption in a preferential liquid solvent comprises absorbing the acetylene in the solvent in an absorption tower, removing those gases which are slightly soluble in acetylene solution to desorption which is effected by means of increased temperatures, decreased pressures, or a combination of both. However, in these conventional methods of absorption and stripping the gas mixture is exposed to conditions under which the poly-formers mentioned above polymerize and provision must be made in all such processes for periodic removal of the polymeric substances to prevent their excessive build-up in the solvent in the absorption and stripping steps. This requires additional, specialized filtration equipment as well as added process steps which are costly with respect to both time and capital expense. Frequently, too, the polymeric material collects in the less accessible parts of the system, fouling up equipment lines, columns and the like. Usually, too, the polymeric material as formed is of such a small particle size that it plugs the filter used for separating it, so that only a small amount can be filtered in each operation and an inordinate length of time is required to wring the filter cake dry.

Methods have been proposed for the removal of the poly-formers from the gas stream but these are cumbersome and expensive since they require additional process steps prior to the usual solvent purification operation which utilize special equipment and significant quantities of special chemicals for treating purposes.

The difiiculties in the prior art processes have been obviated to some extent by the discovery that the polymerization of biacetylene, one of the poly-formers in the acetylene-containing gas, can be inhibited by addition of certain compounds such as hexamethylene tetramine or methylene blue to the solvent in which it is contained.

in a 1:1 ratio incorporated in a solvent effectively inhibits the formation of polymers in an acetylene purifica United States Patent 0 I the solvent in a stripping tower and then subjecting the 2,915,142 1C latented Dec. 1, 1959 tion system wherein. such solvent is employed for the selective absorption of acetylene from an acetylene-containing gas stream and thereafter the resulting solution. is stripped to recover; the acetylene. Surprisingly enough, the combinationof these two inhibitors has an effect far and above-what could be expected from their combined individual action. Obviously, some synergism occurs when these inhibitors are combined which results in a signficant reduction in the amount of polymer formed in the acetylene purification system under consideration.

It is an object of the present invention, therefore, to provide an improved method whereby undesirable polymer growth during the solvent purification of acetylene is inhibited. Other objects and advantages of the invention will be apparent from the following description and the appended claims.

According to the invention, the formation of polymers in an acetylene purification system wherein a solvent is employed for the selective absorption of acetylene from an acetylene-containing gas stream derived from hydrocarbons is eifectively inhibited by, incorporation in said solvent of a minor amount of a mixture of methylene blue and hexamethylene tetramine.

The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner.

EXAMPLE I The testing apparatus consisted of a series of Pyrex gas-washing bottles (SOO-ml. capacity) connected in parallel to gas-inlet and outlet manifolds. The distributing outlets on the gas-inlet manifold all contained steel needle valves and rotameters for regulating and measuring the gas flow. Three sets of tests were conducted. In each set, one bottle was charged with N-methyl pyrrolidone alone while the second was charged with N-methyl pyrrolidone to which methylene blue, hexamethylene tetramine, and a 1:1 mixture of methylene blue and hexamethylene tetramine, respectively, had been added. The solvent was carefully weighed into each bottleso that the same amount, 400 g. (approx. 400 ml.), was used'in each one. An acetylene-containing gas stream was bubbledv through the liquid in the bottles at a fixed rate of about 600 ml. per min. for a period of approximately 48 hr. The gas employed was the effluent stream produced by the oxidation of methane. It was tapped from a process line in an acetylene plant' at a point downstream from the furnace or reactor after the gas had been quenched and filtered to remove any soot present. This gas was then successively passed through a compressor, a pressure regulator and a chilled surge container prior to admittance into the gas inlet manifold.

After the bubblers had been in operation for. 48 hr., the bottles were disconnected and the solutions were weighed. The contents were then transferredto clean glass bottles and analyzed for water and polymer content. Polymer determinations were made as follows: Ten-ml. samples of the solutions were pipetted into 6S-ml. ignition tubes. The solutions were frozen solid by immersingthe tubes in a trichloroethylene-dry ice bath and the tubes were then sealed with an oxygen-natural gas torch. After allowing them to warm up to room temperature, the tubes were placed in an oven maintained at 105 C. for 10 hours. At the end of this time, the tubes were removed, allowed to cool to room temperature, then further cooled to solidify the contents again by freezing, and opened. The opened tubes were allowedvto warm up to room temperature after which .thecontents of each were poured into separate Erlenmeyer flasks and 15 ml. of

. 1 N H was added to each flask. The flasks were heated on a'hot' plate until the mixturestherein were brought to a near boil. The contents of each flask were then filtered on separate tared gooch crucibles, the flasks being rinsed with hot waterand the washings being poured onto the filters. The crucibles were dried in a vacuum whereby the gases dissolved in the solvent are liberated. The gases liberated in each stage or zone are removed from a lower pressure stage and injected into the next higher pressure stage by means of high velocity steam oven at 100 C. for '1 hr., cooled in a desiccator and 5 jets, the gas pressure in the stage of highest pressure weighedto determine the amount of polymer present in being maintained sufficiently high so that there is a flow each sample. of gas from this stage to the bottom of the desorber.

Results of the tests are presented in. Table I. Pure concentrated acetylene is withdrawn from an inter- Table 1 Sample No VIII-1 VIII-9 VIIIel VIII-5 XII-1 XII-11 Inhibitor None ..HMTA None... Methyl- None... Methylene ene blue+ blue. HMIA l Amt. Inhibitor (Wt. Percent of 801- -do... do 0.5 do 0.4 (0.2 ea.). *WiiiiitGained by Solvent (g.) 16.7.-." 16.7. 16.5 11.5 12.2. :Percent H 0 in Solvent:

Initial 0.62--." 0.62--- 0.62. 0.90..... 0.90. Final 3.21-... 3.21.--. 3.21-. 2.0L-.- 2.24. Percent Polymer 1.11-.. 1.11 0.56 1.35..- 0.42. Percent Polymer in Inhibited Soln/ Percent Polymer in Uninhibited Sol'n 0.50 0.31.

1 Hexamethylene tetramine.

These data convincingly demonstrate that a mixture of mediate sectionof the desorber. The liquid solvent from methylene blue and hexamethylene tetramine is a remarkthe bottom of the stage of lowest pressure substantially ably effective agent for inhibiting polymerization of the completely freed of acetylene is pumped to a solvent polymerizable components in acetylene-containing gas dedesorber where it is freed of remaining impurities and rived from hydrocarbons. The mixture of the two comrecycled to the absorption tower. pounds is about more effective than methylene blue In this system where a mixture of methylene blue and alone and 38% more efiective than hexamethylene tetra- 30 hexamethylene tetramine is incorporated in the solvent, mine alone although the total concentration of inhibitor polymerization of the unsaturated polymerizable comused is slightly lower when the mixture is employed. ponents in the gas being treated is held to a minimum.

No appreciable amounts of polymer are deposited in the EXAMPLE H solvent or in the equipment thus obviating the usual A gas mixture obtained by the partial combustion of Processing dhhchhies arising from Plugged P p y methane with oxygen in a suitable furnace containing etcthe following constituents in 1 1 percent, The elfectiveness of the inhibiting mixture is not re- C H 7 5 stricted to those conditions given in the examples. Usu- 40 ally, a1 :1 ratio of methylene blue to he tamethylene tetra- CO mine is employed although other ratios may be used. CO In general, only minor quantities of the inhibiting mixture H 2 are required although the amount used depends somewhat on the amount of poly-formers present in the gas stream ig biacetylene gzy P y p y and the degree of inhibition desired. Concentrations of ers e 1 4 the mixture in the range from about .01 to about 5% by weight of the solvent are suitable, withconcentrations is compressed and fed under pressure into the middle in the range from about to about 1% being Preferredof an absorption tower. This is a tower containing trays, The Stabilizing 0f inhibiting miXhlfB y h course, he pans, or ferrules of any suitable type. The gaseous mixemployfid h a concemrflhoh F ture ascends into the upper portion of the tower coming The inhibiting InlXllllTfi 0f the IDYGHYIOH 1S 1101: restricted into contact therein with descending liquid N-methyl pyrto 1156 y with Nrmethyl py h f Set out 111 l' l done containing in Solution about 0.5% by Weight of examples above. It may be used to inhibit polymerization a 1:1 mixture of methylene blue and liexamethylene tetrain other like Organic Solvents Such as hlliyfolhctonei for i d i d at h top f h tower As h two example. In any solvent system wherea selective solvent streams of ascending vapor and descending liquid come is used for the recovery f acetylene from a gas Stream into contact, the acetylene in the gas dissolves in the N- containing it in comblhahoh With other gases including methyl pyrrolidone so that by the time the liquid solvent P lY Y ahshfptioh in the Solvent and Subsequent reaches the bottom of the absorber it is saturated with pp g j hh i polymefilatih'n 0f Suhh Q Y- acetylene. It contains also small amounts of the other 15 efiechvely lllhlhlted y the Presence of amlhol' amount volatile constituents which are slightly soluble such as of mlxhlfe of methylene hhlh and hexameihylehe tetracarbon dioxide and ethylene. An overhead gas essentially mine in the Solvent T0 inhibit Polymer fihfmatioll free of acetylene is Withdrawn from the top of the abihg the absorption-Stripping Operation, the Inhibiting lhiX- sorber. ture can be added directly to the solvent feed stream, or

Th l i of acetylene f h bottom f h it can be introduced as a solution of the desired concensorber is introduced into a desorber where the pressure tration at some Point in the absorption System The of the gas in contact with the solution is reduced with hibiting power of the mixture of methylene blue and the result that the major portion of the less soluble nonhexamethylene tetramine is not afiected by continual reacetylenic gases is flashed ofi. These less-soluble gases use; hence, recirculation of the solvent may be safely are returned to the scrubber for recovery of any acetylene practiced with the inhibiting efiect of the additive mixcontained in them. The acetylene-saturated solution of ture of the invention suffering no significant decrease in N-n ethyl pyrrolidone leaving the desorber passes through efiectiveness.

a heater wherein its temperature is raised and is then What is claimed is: copducted into a series of stages or zones wherein the 1. In a process for the recovery of acetylene from an solvent is subjected to progressively lower pressures and acetylene-containing gas stream derived from hydrocarsolvent being non-reactive with a 1:1 mixture of methylene blue and hexamethylene tetramine under the conditions employed.

2. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with a solvent for selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in the solvent employed a 1:1 mixture of methylene blue and hexamethylene tetramine, said mixture being present in an amount in the range from about 0.1% to about 1% by weight of the solvent, said solvent being non-reactive with a 1:1 mixture of methylene blue and hexamethylene tetramine under the conditions employed.

3. In a process for the recovery of acetylene from an 25 2,664,997

acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in said N-methyl pyrrolidone solvent a 1:1 mixture of methylene blue and hexamethylene tetramine, said mixture being present in an amount in the range from about 0.01% to about 5% by weight of the solvent.

4. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in said N-methyl pyrrolidone solvent a 1:1 mixture of methylene blue and hexamethylene tetramine, said mixture being present in an amount in the range from about 0.1% to about 1% by weight of the solvent.

References Cited in the file of this patent UNITED STATES PATENTS Eck Jan. 5, 1954 

1. IN A PROCESS FOR THE RECOVERY OF ACETYLENE FROM AN ACETYLENE-CONTAINING GAS STREAM DERIVED FROM HYDROCARBONS WHEREIN THE ACETYLENE-CONTAINING GAS STREAM IS CONTACTED WITH A SOLVENT FOR SELECTIVE ABSORPTION OF ACETYLENE THEREFROM AND SAID SOLUTION IS THEREAFTER STRIPPED TO RECOVER ACETYLENE THEREFROM, THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN THE SOLVENT EMPLOYED A 1:1 MIXTURE OF METHYLENE BLUE AND HEXAMETHYLENE TETRAMINE, SAID MIXTURE BEING PRESENT IN AN AMOUNT IN THE RANGE FROM ABOUT 0.01% TO ABOUT 5% BY WEIGHT OF THE SOLVENT, SAID SOLVENT BEING NON-REACTIVE WITH A 1:1 MIXTURE OF METHYLENE BLUE AND HEXAMETHYLENE TETRAMINE UNDER THE CONDITIONS EMPLOYED. 